A standard electrical plug has either two or three prongs attached to the plug. In a two-prong plug, one prong is live, and carries the current from the source to the load. The other prong is neutral, which returns the current to the source. The live prong and the neutral prong may be of the same size, or they may be different sizes to ensure that the live prong and the neutral prong match the corresponding receptacles in a wall outlet.
A three-prong plug is similar to a two-prong plug in that it includes a live prong and a neutral prong. However, a three-prong plug also includes a ground prong. The ground prong connects the electrical device to earth ground, which is intended to protect against insulation failure of the connected device, such as when a live wire unexpectedly delivers current to an exposed metal part of the device. The ground connection permits such current to be safely handled. Connections to ground also limit the build-up of static electricity on parts of the device, which can be important in the presence of flammable products or when repairing the device. In some power transmission circuits, the earth itself can be used as one conductor of the circuit, saving the cost of installing a separate return conductor.
A three-prong plug provides many advantages over a standard two-prong plug. However, the corresponding three-prong outlet, necessary when using a three-prong plug, has only recently become commonly used in the construction of housing. It was common practice in older homes to install two-prong outlets instead of the grounded three-prong outlets. In many cases, the cost of upgrading all of the electrical outlets to three-prong outlets is prohibitive, partly due to the requirement that a continuous grounding wire would need to be run to all outlets.
Rather than upgrade the electrical outlets, many users of electrical devices use a ground lifting plug. A ground lifting plug is a plug with a three-prong receptacle on one side, and a two-prong plug on the other side. This turns a three-prong plug into a two-prong plug, allowing a user to install an electrical device with three-prongs, in older homes. The problem with using a ground lifter is that a user would need a ground lifter for each three-prong plug he/she wants to add into a two-prong outlet. Ground lifters are generally of an unattractive gray color, and add significant length to the plug, preventing items from being placed flush against the wall.
A second significant problem with typical three prong plugs used today, is that the ground prong tends to be less sturdy than the live prong and the neutral prong. The live and neutral prongs are of a thin rectangular shape. They can be bent slightly along a single axis without breaking, and the shape prevents any bending in a perpendicular axis. The ground prong, however, is of a generally cylindrical shape, and can bend, and even break off by movement in any direction. For example, a user removing a three-prong plug from an outlet, may inadvertently break the ground prong by removing the three-prong plug at too steep of an angle. If the ground prong breaks, the entire plug, and or wire plug set would need to be replaced. This can be very expensive, time consuming, and difficult to do.
U.S. Pat. No. 3,531,757 to Alden discloses a plug with a user-replaceable ground prong, which has a groove that is held in place by two indentations inside the body of the plug by means of an interference fit. A disadvantage of this device is that the indentations are susceptible to wear and tear if the ground prong is removed and replaced in the body and/or if the plug is removed and replaced from an outlet on a regular basis. The indentations may lose the ability to hold the ground prong, which may result in the ground prong remaining inside of the electrical outlet when the plug is removed. Additionally, if the user bends the plug at too steep of an angle when removing the plug from an outlet, the ground prong may break, and it is possible that the ground prong will break off at a portion inside of the plug body. This would render the plug useless as a user would not be able to remove the ground prong from the plug body to be able to replace it.
U.S. Pat. No. 6,419,504 to Nelson discloses a plug with a ground conductor that can be slid in and out of the plug body. A switch on the plug body allows the user to lock the ground prong in place in either an extended position outside of the plug body or a retracted position inside of the plug body. A disadvantage of this device is that the ground prong is not user replaceable. If the ground prong were to break, easy replacement of the ground prong it is not possible, rendering the plug useless for its intended purpose.
U.S. Pat. No. 3,739,317 to Wise discloses a plug with a reconfigurable ground prong. In one embodiment, the ground prong is retractable and has grooves that engage an external fastener in either an extended or retracted position. A clear disadvantage of this design is that, if the ground prong were to break, it would most likely break at the innermost groove, which is inside the plug body, making it extremely difficult to remove the remainder of the prong for replacement.
Additionally, prongs having a generally cylindrical shape are used in a variety of other circumstances. For example, plugs used in many European countries have prongs that are generally cylindrical and susceptible to breakage. Such plugs are also expensive to replace in the event of breakage of one of the prongs.
What is desired, therefore, is an electrical plug with a replaceable prong that that can be easily removed and replaced in the event of breakage. More particularly, what is desired is an electrical plug with a replaceable ground prong that allows the plug to be easily used in either a two-prong or a three-prong configuration. It is also desired that the plug be configured such that, in the event of breakage, the remainder of the ground prong can still be easily removed from the body of the plug. It is further desired that the plug be inexpensive and simple to manufacture.